1. Field of the Invention
The present invention relates to a piezoelectric actuator using a piezoelectric device.
2. Description of the Related Art
Various structures of piezoelectric actuators have been conventionally proposed. For example, a piezoelectric actuator disclosed in Kokai (Japanese Unexamined Patent Publication) No. 7-335952 is provided with a piezoelectric device whose periphery is coated with an outer dense film (resin mold) of a uniform thickness.
Also, a piezoelectric actuator disclosed in Kokoku (Japanese Examined Patent Publication) No. 6-28832 has a sealed structure in which an extensible case is arranged on a side surface of a piezoelectric device.
Further, Kokai No. 2-250678 discloses a piezoelectric actuator in which a space between a piezoelectric device and a case is hermetically filled with silicon oil.
It is still difficult to apply conventional piezoelectric actuators to a device whose application requirements are strict, such as an injector (fuel injection system), for, for example, an internal-combustion engine for a motor vehicle. Namely, for example, in an injector, its actuator must be small and of quick-response.
To establish electrical insulation between the piezoelectric device and the case, a certain insulating distance must be provided therebetween. From the viewpoint of (reduction in diameter) miniaturization, insertion of resin, etc. therebetween, contributes to miniaturization much more than simple provision of the insulating distance.
However, if electric insulation is established by the resin mold as mentioned above, latter prevents extension and contraction of the piezoelectric device, thus resulting in reduction of the force to be produced by the piezoelectric device. Also, the resin mold can be cracked by the repeated extending-and-contracting operations so that the electrical insulation can be deteriorated. Especially in the injector, which requires quick-responsibility, the crack production probability is increased.
The invention is aimed at provision of a piezoelectric actuator which can be easily miniaturized and which exhibits high electric insulation, in view of drawbacks in the prior art.
According to an aspect of the present invention, in a piezoelectric actuator using a laminated piezoelectric device having alternately layered piezoelectric layers and electrode layers, provision is made of a metal case provided on the outside of the side surface of the piezoelectric device and an insulating member separate from the piezoelectric device, provided between the piezoelectric device and the case.
The mode of operation and the effect of the present invention will be explained below.
In the present invention, the insulating member is provided between the piezoelectric device and the case. The insulating member is made of a piece separate from the piezoelectric device. Therefore, when the extension and the contraction of the piezoelectric device occurs, the movements are little restricted by the insulating member. Consequently, the force produced by the piezoelectric device is little reduced by the insulating member.
Further, unlike the resin mold, no crack of the insulating member occurs due to the extending-and-contracting movements of the insulating member caused by the extending-and-contracting movements of the piezoelectric device. As a result, high electric insulatability of the insulating member can be maintained. With the presence of the insulating member, the distance between the metal case and the piezoelectric device can be decreased, as compared to the absence of insulating member. Thus, the piezoelectric actuator can be miniaturized.
Therefore, according to the present invention, a piezoelectric actuator having a high electric insulatability, which can be easily miniaturized is obtained.
According to another aspect of the present invention, it is preferable that the insulating member be a cylindrical sleeve made of resin or paper. In this aspect, since the insulating member is in the form of a sleeve, the insulating member can be arranged merely by inserting the piezoelectric device in the sleeve, so that the piezoelectric actuator can be easily assembled.
According to still another aspect of the present invention, the insulating member can be in the form of a resin or paper sheet wound around the piezoelectric device. In this aspect, the insulating member is wound when assembling, so that an optimum shape of insulating member corresponding to the cross-sectional shape, size, etc., of the piezoelectric device, can be easily formed.
According to yet another aspect of the present invention, it is preferable that at least the overlapped portions of the wound sheet material be adhered to each other. Namely, the sheet member is preferably coated with an adhesive layer or is preferably made of self-adhesive resin so that the overlapped portions are adhered to each other. In this alternative, no displacement of the overlapped portions of the insulating member takes place in the case, thus resulting in a stable arrangement.
Note that if the adhesive-coated sheet is wound around the piezoelectric device, the sheet may be adhered to the piezoelectric device. In this case, the adhesion can be accepted so long as the piezoelectric device can be extended and contracted. Rather, the adhesion contributes to a stable arrangement of the insulating member.
According to another aspect of the present invention, it is preferable that the maximum thickness of the insulating member be no more than 0.3 mm. In this aspect, the heat radiation efficiency of the insulating member can be enhanced.